comp: Render subimage properties.

This patch passes the offset and extent properties to the layer shader by extending the uniform. The fragment shader stage now also receives the transformation uniform, which contains a has_sub boolean to distinguish if the properties are set, so between projection and quad layers.

To avoid color bleeding the subimage sampling happens on a global pixel coordinates basis as ivec2 using the GLSL texelFetch function. Projection layers will be sampled as before.

In addition I have 2 patches that clean up the layer renderer a bit.